Preparation and analysis of superconducting Nb-Ge films

The dependences of $T_c$, resistivity, resistance ratio, and structure on chemical composition and sputtering conditions for Nb-Ge films have been studied. The chemical composition, impurity content, and x-ray structure were obtained using Rutherford backscattering, nuclear techniques, and x-ray diffraction. Although $T_c$ varies with composition, it is not found to be critically dependent upon exact stoichiometry; the Nb/Ge ratios vary by ∼ 13% (2.6 to 3) for films with ∼ 23-K onsets and by ∼ 40% (2.2 to 3.3) for films with ∼ 20-K onsets. For compositions similar to the bulk, the films have comparitively much higher $T_c$'s and smaller lattice parameters. X-ray results show the films to contain predominately $A-15$ phase (except for $\frac{\mathrm{Nb}}{\mathrm{Ge}}\lesssim 2.5$) with lattice parameters varying from 5.15 Å for Nb-rich low-$T_c$ films to 5.12 Å for Ge-rich films. Several percent of oxygen and carbon occur in low-$T_c$ amorphous films deposited at 650 °C but this is considerably reduced in high-$T_c$ films made simultaneously at ∼ 750 °C. No argon was found and the nitrogen content was generally less than 1%. No correlation of high $T_c$'s and impurities was found. The optimum deposition temperature and resistivity are lowest, and the resistance ratio highest for Nb/Ge ratios somewhat below 3/1. A simple correlation of $T_c$ and resistance ratio is reported which is largely independent of all sputtering conditions and composition and which suggests that slightly higher $T_c$'s may be possible. Negative bias was found to be detrimental to $T_c$ while positive bias had relatively little effect. Magnetic-field-assisted sputtering led to significant increases in the sputtering rate and the optimum deposition temperature.